Field of the Invention
The present disclosure relates to a field-effect transistor, a display element, an image display device, and a system
Description of the Related Art
Flat panel displays (FPDs), such as liquid crystal displays (LCDs), organic electroluminescent (EL) displays (OLEDs), and electronic paper, are driven by a driving circuit including a thin film transistor (TFT) obtained by incorporating amorphous silicon or polycrystalline silicon into an active layer. FPDs have been required to have a further increase in size, higher definition, and high-speed driving power. Therefore, there is a need for provision of a transistor having the following properties: high carrier mobility; high on⋅off ratio; and such favorable switching property as rapid rising from the off-state to the on-state.
However, the TFTs obtained by incorporating amorphous silicon (a-Si) or polycrystalline silicon (i.e., in particular, low-temperature poly silicon: LTPS) into an active layer have advantages and disadvantages. Therefore, it is difficult to satisfy all the requirements for the TFTs at the same time.
For example, a-Si TFTs have the following disadvantages. The a-Si TFTs have insufficient mobility for driving a liquid crystal display (LCD) having a large area at high speed and have a large shift of threshold voltage when continuously driven. LTPS-TFTs have high mobility but have large variation in threshold voltage due to a process for crystallizing an active layer through excimer laser annealing. Therefore, a size of mother glass for a mass-production line cannot be enlarged, which is problematic.
Accordingly, there has been proposed a InGaZnO4 (a-IGZO) which can be formed into a film at room temperature and exhibits mobility equal to or higher than mobility of a-Si in an amorphous state (see K. Nomura, 5 others, “Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors”, NATURE, VOL 432, No. 25, November, 2004, pp. 488-492). The proposed material has been a trigger for actively studying amorphous oxide semiconductors having high mobility.